Charge fluxes and changes in electronic structures as the origin of infrared intensities in the ground and excited electronic states

A theory for describing the mechanism giving rise to infrared (IR) intensities in the ground and excited electronic states is presented, which is applicable to modes representing the vibrational motions in the direction of the transition between two resonance structures having different dipole moments, such as strongly IR active modes characteristic of charged polyenes, protonated conjugated Schiff bases, and peptides. A simple Hamiltonian based on a two-state model is used. The relationship between charge fluxes giving rise to IR intensities and changes in electronic structures is examined in detail. The results derived from the simple model Hamiltonian are compared with those of ab initio molecular orbital calculations, which are regarded as solutions of more realistic Hamiltonians, for the pentadienyl cation, the heptatrienyl cation, the 2,4-pentadienylideneammonium cation, and N-methylacetamide in the ground and excited electronic states.